Normally, semiconductors, electronic circuits, and electronic devices, among others, have various electronic components fused and fastened to a substrate using solder, to obtain electric conductivity. However, the conventional solders are an alloy of Sn and Pb, and because a usage of Pb is becoming prohibited as an environmental protection measure of the recent years, Pb-free substitute solders are being developed to replace said conventional solders. The melting point of a eutectic solder of Sn and Pb is 183° C., and the melting point of an Sn/Ag/Cu solder that is a conventional substitute solder is 217° C. When soldering is done on a resin substrate, because the heat resistance of resin is low, the melting point of a conventional substitute solder can be too high, causing damage to the resin substrate. There has been a demand for a low temperature solder.
As for the characteristics of a substitute solder, naturally it should not contain Pb, and moreover, the metalization temperature should be low, but, in addition, it is desirable that the safety is high, it is not corrosive, it is capable of bonding under no-load (using only self-weight, and applying no further load) just like a solder, and moreover, the bond strength is high. Composite metal nanoparticles of Cu, Ni, Ag, and others are being developed as a material meeting this expectation.
Next, nanometal pastes were produced, in which a viscosity imparting agent and a solvent were mixed into these metal nanoparticles. For example, the Japanese Patent Laid-Open No. 2007-297671 has been published as Patent Document 1. In this document, a nanometal paste is suggested, in which the average particle diameter of the primary particles of silver nanoparticles is less than or equal to 200 nm, and a sheet-like structure having remnants of the primary particles of these silver nanoparticles is made to be the metallic component.
The present inventors made nanometal pastes using commercially available silver nanoparticles whose average particle diameter was less than or equal to 200 nm, and did experiments to bond a copper disc with diameter of 5 mm to another with diameter of 10 mm. That is to say, a nanometal paste layer of 300 μm thickness was applied and formed on the surface of a copper disc with diameter of 5 mm, a copper disc with diameter of 10 mm is adhered to this paste surface, and both discs were bonded by sintered at a firing temperature of 350° C. and sintering said paste layer into a metal layer. After cooling, the shear bond strength that is the fracture limit was measured by separating and shearing both by applying an external force toward the surface direction of said copper disc with diameter of 5 mm. However, no matter how often it was repeated, bond strength at a level of only 0.1-1 MPa was obtained under no-load. The bond strength that the present inventors require is at least greater than or equal to 10 MPa, where a semiconductor is damaged. In particular, in an electronic component used under a high temperature and an electronic component used under strong motion, a high bond strength greater than or equal to 10 MPa, in which the joining portion is not damaged but the semiconductor is damaged, is required. By the nanometal pastes described above, it is absolutely impossible to be put into practical use.
An improvement of silver nanoparticles is needed if high bond strength greater than or equal to 10 MPa is to be stably obtained. Usually, for nanoparticles to fit into the definition, the particle diameter of said silver nanoparticles must be less than or equal to 100 nm. The average particle diameter of said silver nanoparticles is 200 nm, and they have a flaw that they are too large to be a metallic component. Moreover, because such silver nanoparticles mutually aggregate easily, it is estimated that the silver nanoparticles aggregated mutually and became large lump-like silver particles in the previously described nanometal paste, and as a result, even when it was fired, many gaps were caused in the bonding metal layer, and the bond strength deteriorated. To prevent aggregation, silver nanoparticles that can be monodispersed in an organic solvent must be used, and for this, they need to be composite silver nanoparticles in which an organic coating layer is formed circumferentially. The reason is because organic coating layers repel mutually, and therefore composite silver nanoparticles are stable in solvent, and they monodisperse. Therefore, to begin with, the present inventor started to develop composite silver nanoparticles.
The present inventors searched, and as a result, it was found that Patent Documents 2-9 shown subsequently are publicly known as patent documents concerning composite metal nanoparticles, and by correcting the flaws in these publicly known documents, the present invention was completed.
First, the Japanese Patent Laid-Open No. 2008-161907 was published as Patent Document 2. This patent application publication discloses a bonding material in which organic-coated metal particles with an average particle diameter of less than or equal to 100 nm, and organic-coated metal particles with a particle size distribution of 100 nm-100 μm, are mixed. The organic-coated metallic material is composite metal particles in which an organic coating layer surrounds a metal core, and it has characteristics in common with the composite metal nanoparticles used in the present invention. The main point of this patent application publication states that the bond strength increases because the organic-coated metal particles less than or equal to 100 nm fill the gap between the metal particles greater than or equal to 100 nm, but that the details of the mechanism are unknown. About the particle diameter relation between the two kinds of metal particles, it is only described that they are metal particles greater than or equal to 100 nm, and metal particles less than or equal to 100 nm. Because the mechanism is unknown, it is a fact that there is no further mention about the particle diameter relation at all.
The Japanese Patent Bulletin No. 3205793 (Japanese Patent Laid-Open No. 10-183207) was published as Patent Document 3. Silver organic compounds (particularly, silver organic complexes) were chosen as starting materials. Under the inert gas atmosphere from which air was blocked, said silver organic compound was heated at a temperature greater than or equal to the decomposition start temperature and below the complete decomposition temperature, and composite silver nanoparticles were manufactured, in which the organics originating from said silver organic compound were made to be the coating layer around the circumference of silver cores that had been decomposed and reduced. This preparation method is a solid-gas reaction. The particle diameter of the silver cores is 1-100 nm, and therefore they are referred to as composite silver nanoparticles. Specifically, when 100 g of solid silver stearate was put in a flask under nitrogen gas stream, and heated at 250° C. for 4 hours, composite silver nanoparticles having an organic coating layer of stearate group around the circumference of a silver core with 5 nm particle diameter were produced.
Because in said manufacturing method, a solid body of silver stearate is heated without a solvent, the produced composite silver nanoparticles are difficult to disperse, and there is a flaw that a large number of composite silver nanoparticles become large secondary particles that are bonded in a lump-like state. Moreover, the production temperature is high, namely 250° C., and it can be seen that the metalization temperature of the composite silver nanoparticles is extremely high, namely 220° C. Silver nanoparticles whose production temperature is high also have a high silverization temperature. The melting point of a regular Sn—Pb solder is 183° C., and considering that the desired metalization temperature of composite silver nanoparticles is less than or equal to 200° C., said metalization temperature (silverization temperature) is too high at 220° C., and it is difficult to use them as a substitute low-temperature solder. It is thought that the high metalization temperature is caused by the very large particles in a lump-like state, and also by the high decomposition temperature of the stearate group. Moreover, the inventors have confirmed that said silver core is not a single crystal, but simply an atom aggregate or a polycrystal. When the silver cores are polycrystals or disordered, electronic and heat scatterings are caused at a large number of grain boundary surfaces, and as a result, it becomes a cause for lowering of the electric and heat conductivities.
Next, Japanese Patent Laid-Open No. 2003-342605 bulletin was published as Patent Document 4. Said Patent Document 4 is an invention in which one of the present inventors participated in as an inventor. A metal organic compound was dissolved or dispersed in an organic solvent or water, and successfully produced composite silver nanoparticles coated with the organics originating from said metal organic compound. This preparation method is a solid-liquid reaction. Moreover, when this composite silver nanoparticles were observed under a high resolution transmission electron microscope, lattice images were observed on the silver cores, and it was confirmed that they were single crystal silver cores. It is thought that, based on the solid-liquid reaction method, the metal organic compound dissolved and dispersed into the solvent as molecules, said molecules were reduced to precipitate silver atoms, and they became single crystals through recombination between the silver atoms. That is to say, it is thought that the single crystal characteristics are caused by intermolecular reaction. Because the silver cores are single crystals, they have an advantage that the electric and heat conductivities are high. However, as for the silverization temperature, it is written in [0076] that the composite silver nanoparticles with stearate coating were heated at 250° C. for 10 minutes. In other words, a weak point of Patent Document 4 is that the silverization temperature is very high at 250° C. A reason the silverization temperature is high is that the decomposition temperature of the carboxylates that comprise the coating layer is high, for they start from silver organic compounds such as silver acetate, silver hexanoate, and silver octanoate, among others. A further measure is needed to make the metalization temperature less than or equal to 200° C.
Thus, the WO00/076699 bulletin was published as Patent Document 5. One of the present inventors is one of the inventors of this international application publication. Multiple inventions are disclosed in this publication, but among them, a method for processing a metal inorganic compound by means of a surfactant was disclosed for the first time, and a pathway was opened for using a metal inorganic compound as the starting material. That is to say, it consists of the first step, where an ultra-fine particle precursor is formed by making a colloid out of a metal inorganic compound in an nonaqueous solvent by means of surfactant, and the second step, where a reducing agent is added to this colloidal solution, and said ultra-fine particle precursor is reduced, generating composite metal nanoparticles on which a surfactant shell is formed as a coating layer around the circumference of a metal core.
Because a metal inorganic compound is dissolved in a nonaqueous solvent in said method, it has a characteristic that the produced composite metal nanoparticles disperse within the nonaqueous solvent, and therefore it is difficult for them to be in a lump-like state. However, the embodiments disclose copper oleate, silver abietate, silver acetate, nickel oleate, diethyl hexane indium, copper acetate, and silver stearate, and only organometallic compounds are put into use. Moreover, it was found that the metalization temperature of the composite silver nanoparticles produced from silver stearate was high, at 220° C. A further measure to make the metalization temperature less than or equal to 200° C. is need. To make it have even higher characteristics than Sn—Pb solders, even further effort is required for making the metalization temperature less than or equal to 150° C. Moreover, because a determination of the single crystallinity/polycrystallinity of the silver cores was not made in Patent Document 5, the quality of the electrical and thermal conductivities of the composite metal nanoparticles cannot be determined.
Under the above circumstance, the WO01/070435 bulletin was published as Patent Document 6. This international application publication discloses composite metal nanoparticles in which a coating layer is formed, comprising of organic compounds with carbon number of 4 or higher and including alcohol hydroxyl group, around the circumference of a metal core with 1-100 nm particle diameter that is obtained from a metal salt. Moreover, as an organic compound including a functional group with an adsorptive property, a higher alcohol whose carbon number is greater than or equal to 6 is described.
Furthermore, the WO2005/075132 bulletin was published as Patent Document 7. This publication discloses composite metal nanoparticles whose central part comprises a metal core, and having around it a coating layer of organics whose thermal desorption start temperature is greater than or equal to 140° C. and less than 190° C. As the manufacturing method, it is described that an inorganic metal salt is made to coexist with an organic material, the inorganic metal salt disintegrates, metal cores are formed, and composite metal nanoparticles are produced, in which coating layers of organic matter is formed around the circumference of said metal cores. Also, composite metal nanoparticles are disclosed, in which a coating layer of organic matter is formed around the circumference of an inorganic metal salt or an inorganic metal compound produced by decomposition.
The Japanese Patent Laid-Open No. 2007-95510 bulletin has been published as Patent Document 8. In claim 1 of Patent Document 8, an electroconductive paste is disclosed, comprising composite metal nanoparticles composed of metallic cores consisting of a metal component originating from a metal salt expressed by a chemical formula of (R-A)n-M, organic coating layers originating from said metal salt, and an organic solvent. R is a hydrocarbon group of carbon number 4-9, A is COO, OSO3, SO3 or OPO3, and M is silver, gold or a platinum group. Therefore, composite silver nanoparticles are included.
The Japanese Patent Laid-Open No. 2004-107728 bulletin is disclosed as Patent Document 9. In Claim 1 of Patent Document 9, composite metal nanoparticles are described, that contains organic coating layers whose main components are C, H and O are placed in the circumference of metal cores with an average particle diameter of less than or equal to 100 nm. It is described that these organic coating layers are produced from an organic acid metal salt.
[Patent Document 1] Japanese Patent Laid-Open No. 2007-297671 bulletin
[Patent Document 2] Japanese Patent Laid-Open No. 2008-161907 bulletin
[Patent Document 3] Japanese Patent No. 3205793 (Japanese Patent Laid-Open No. 10-183207 bulletin)
[Patent Document 4] Japanese Patent Laid-Open No. 2003-342605 bulletin
[Patent Document 5] WO00/076699 bulletin
[Patent Document 6] WO01/070435 bulletin
[Patent Document 7] WO2005/075132 bulletin
[Patent Document 8] Japanese Patent Laid-Open No. 2007-95510 bulletin
[Patent Document 9] Japanese Patent Laid-Open No. 2004-107728 bulletin